The long-term objective of the proposed study is to determine whether elevated ambient levels of O3 typical; of many large US cities, enhance the effect of aeroallergens on asthma status among allergic individuals. The specific aims of the research are to examine the interactive relationship of daily levels of O3 and outdoor aeroallergens to asthma symptoms, Peak Expiratory Flow Rate (PEFR) measurements, and bronchodilator use among mild to moderate asthmatics in San Diego County, from April through September in 2 panels. A third panel of children ages 6-13 exposed to high levels of O3, but low particulate levels, will be followed from April through June. The relationship of atmospheric acidity to daily asthma status will also be examined; this objective may gain considerable importance depending on levels of acidity found in the study region. Allergic status will be determined with a confirmed seasonal history, and positive response to specific allergen skin tests. Diary entries will be made for the severity of asthma symptoms (wheeze, cough, dyspnea, sputum production and chest tightness). Outdoor environmental measurements will include O3, No2, PM10, PM25, particle- and vapor-phase atmospheric acidity, pollen and mold counts, minimum and maximum temperature and relative humidity, and Santa Ana weather conditions. Indoor and occupational exposures will be assessed with initial questionnaires. Environmental tobacco smoke exposure, and use of gas stoves/ovens, heaters and air conditioners will be recorded daily in diaries. Specific times spent outdoors and relevant exercise levels will be recorded for exposure-adjusted models. Analyses will include rigorous control for temporal confounding, aggregate and nonaggregate dependent variable models, and additive and multiplicative models for statistical interaction. There is now experimental evidence that suggest O3 and other air pollutants may enhance the ability of aeroallergens to exacerbate asthma in allergic individuals. The relevance to public health of this potential effect modification has not been fully investigated. The proposed epidemiologic study is intended to address this issue in a free-living population of asthmatics most likely to be adversely affected by interactive relationships of ambient air pollutants and aeroallergens.